For a conventional light-emitting diode (LED) structure, the device efficiency rises as the operating current increases, but when the operating current increases and reaches a certain value, the device efficiency declines. Such efficiency declination phenomenon is called “efficiency droop”. As shown in FIG. 1, the external quantum efficiency of the LED device reaches the highest point at a current density of about 10 A/cm2, and when the current density increases, the external quantum efficiency of the device declines dramatically. This efficiency droop phenomenon limits the efficiency of the LED under a large current.
The cause of this efficiency droop phenomenon is still inconclusive, but it is generally believed that it is due to the mismatch of mobility of the electrons and holes in a high-current operation, so the electrons in the multi-quantum well (MQW) of the light-emitting layer overflows easily to the p-type semiconductor layer, and electrons are unevenly distributed in the multi-quantum well. Most of the electrons are concentrated in one or several quantum wells near the p-type semiconductor layer, so the holes that are injected into the light-emitting layer are insufficient, and the device efficiency declines in a high-current operation.
To solve the problem of the uneven distribution of carriers in the multi-quantum well of the light-emitting layer, a super-lattice active layer is adopted. That is, the thickness of the barrier layer in the multi-quantum well structure of the light-emitting layer is designed to be thin. When the thickness of the barrier layer is so thin that the electrons and holes are able to tunnel through the barrier layer because of the formation of the mini-band in the principles of quantum mechanics, a more uniform distribution of carriers is achieved, which is helpful to solve the above-mentioned efficiency droop problem.